Short answer: a purifier sized for your room, running on high with the door closed, removes most of the airborne particles in about 30 minutes to an hour, and about 90% in under an hour if it changes the air four or more times an hour. A machine that is too small for the room, or running on low, can take two hours or more. The number that decides it is air changes per hour: how many times an hour the machine moves the room's full volume of air through its filters.
The math in one table
In a closed room where the air mixes well, each full air change removes about 63% of the particles that are left. That gives this pattern, with no new pollution coming in:
| Air changes an hour | Removed in 15 min | Removed in 30 min | Removed in 1 hour | Time to about 90% |
|---|---|---|---|---|
| 2 | 39% | 63% | 86% | About 70 minutes |
| 4 | 63% | 86% | 98% | About 35 minutes |
| 5 | 71% | 92% | 99% | About 28 minutes |
| 8 | 86% | 98% | Nearly all | About 17 minutes |
These are ideal figures from the standard well-mixed-room model, worked out by us. Real rooms are slower: air does not mix perfectly, doors open, and dust, smoke or pets keep adding to the air. Treat the table as a best case.
How many air changes your machine gives your room
Austin Air and Airpura both rate their machines by the room size they clean at two air changes an hour. In a smaller room the same machine changes the air more often. With a standard ceiling height:
| Machine | Rated room at 2 air changes | 150 sq ft bedroom | 300 sq ft room | 500 sq ft room |
|---|---|---|---|---|
| Austin Air HealthMate Junior | 350 sq ft | About 4.7 | About 2.3 | About 1.4 |
| Austin Air HealthMate | 750 sq ft | About 10 | About 5 | About 3 |
| Airpura R400 | About 1,000 sq ft | About 13 | About 6.7 | About 4 |
| Airpura R600 | About 2,000 sq ft | About 27 | About 13 | About 8 |
Room ratings are the makers' own; the air change figures are our arithmetic from them. Higher ceilings mean fewer air changes. Ratings are generally given at the top fan speed (the EPA notes that clean air delivery rates are measured at the highest speed), so on a lower setting the times stretch out.
Particles clear faster than smells
The table above is about particles: dust, pollen, dander and smoke particles that a True HEPA filter catches. Smells and chemical gases work differently. Carbon has to hold each gas as the air passes, and many smells keep coming off fabric, furniture and walls for days. Expect a smoke or new-furniture smell to fade over days, not minutes. More in HEPA vs activated carbon.
How to make it work faster
- Close the door and windows. A purifier cleans the room it is in, not the house.
- Start on high for the first half hour, then turn it down to a speed you can live with.
- Keep it a foot or two from the wall and out of corners, so air reaches the intake. See where to put an air purifier.
- Size the machine for more room than you have. The EPA's rule of thumb for smoke works out to about five air changes an hour. The method is in what size air purifier you need.
- Leave it running. Once you turn it off, the room starts filling back up.
Common questions
How long should I run an air purifier each day?
All the time, in the room you use most. It only cleans while air passes through it, and new particles come in all day.
How do I know it is working?
A particle counter shows it best: the count drops within minutes of turning the machine on high in a closed room. Without one, less dust on surfaces and a fading smell over a few days are the usual signs.
Why does my room still smell after a few hours?
Smells come off fabric and furniture for days, and carbon removes them only as the air passes through. A machine with more carbon, measured in pounds, keeps up longer. See how much activated carbon an air purifier needs.
Does running it on low still help?
Yes, more slowly. On low, a machine sized for a bigger room than yours can still change the air several times an hour, which is why oversizing works well in bedrooms.
Not sure your machine is big enough? Browse our best sellers, or email the room size to support@freshhomeair.com and Nolan will answer himself. Every machine ships free with a 30-day home trial.
